Present invention relates to an improved process for preparing compressed yeast. More particularly, it relates to a process which improves the filterability of the yeast and results in a product having improved properties.
The largest use for viable yeast is for baking purposes, and is supplied to bakeries and consumers in two principal forms, i.e., active dry and compressed. Compressed yeast typically contains about 30% yeast solids on a weight basis, whereas active dry yeast typically contains less than 10%, and generally from about 5 to about 8% moisture. Active dry yeast may be produced from a suitable compressed yeast by any of the several processes known in the art; for example, a process known as the "spaghetti process" which involves extruding compressed yeast in spaghetti form and drying it under controlled conditions on a moving belt.
Commercial yeast production typically entails propagation in a plurality of stages to attain the high degree of purity required for baking purposes. Typically, propagation starts with seed stages and finishes with growing in fermentors of commercial scale. The yeast is grown under aerobic conditions by the addition of large volumes of air to the growth medium. Carbohydrates and nitrogen sources are continuously incorporated into the yeast mash in the last stages of propagation. The temperature of the growth media is maintained in the range where optimum growth of the yeast occurs.
After propagation of the yeast, the yeast is separated from the other constituents of the growth media such as by centrifugation to produce a cream yeast which typically contains about 18% solids. The cream yeast is then washed by redispersing it in water and reconcentrating it to form a purified cream yeast suspension.
In order to produce a commercial compressed yeast product, a cream yeast suspension is subjected to filtration in order to increase the solids content and to reduce the plasticity to improve shaping and forming of the yeast in subsequent stages. Traditionally, the filtration equipment used in the industry includes filter presses or suction filters, with rotary vacuum filters being particularly preferred. Filter presses require the application of high pressures, on the order of 12 atmospheres or more, to force the liquid phase of the yeast suspension through a filtering medium which usually consists of a canvas or fiber cloth. The suction filters function by creating an area of reduced pressure on one side of the filtering medium, usually by the use of a vacuum pump, which causes the liquid phase of the yeast suspension to be drawn through the filtering medium. Commercially, the maximum practical pressure differential that can be obtained on rotary vacuum filters is less than one atmosphere.
In order to increase the solids content of the compressed yeast, Kuestler et al in U.S. Pat. No. 2,947,668, suggest adding an osmotically-active compound, such as sodium chloride to the yeast cream prior to filtration to withdraw intracellular water from the individual yeast cells. The cream yeast suspension is allowed to remain in contact with the osmotically-active compound until the exudation of water due to the difference in osmotic pressure between the intracellular water and the extracellular water is completed. This usually requires several minutes, after which period of time the cream yeast suspension is subjected to vacuum filtration. To reduce the level of osmotically-active compound such as salt in the final product, while not permitting rehydration of the individual yeast cells, the yeast is washed while on the rotary vacuum filter drum. This enables displacement of the osmotically-active material from the extracellular water in an extremely short period of time without permitting the wash water to remain in contact with the yeast cells long enough for reabsorption of any significant amount of water. The contact is disclosed to be from about 0.5 to about 1.5 seconds, in many cases, depending upon the thickness of the yeast layer.
The remainder of the process disclosed by Kuestler et al is then similar to the prior art which existed at that time. The principal difference being that where the vacuum filtration is capable of withdrawing sufficient extracellular water to achieve a slightly less than adequate degree of firmness, at least a portion of the remaining extracellular water is reabsorbed into the individual yeast cells after filtration to result in increased firmness and decreased plasticity. The patent emphasizes, however, that to achieve this desired result, the step of washing the yeast free of the osmotically-active compound must be done extremely rapidly so that a difference in osmotic pressure between intracellular water and the extracellular water remains after filtration. If contact with the washing water is maintained for greater than this minimal time, extracellular water will be taken into the individual yeast cells, driven by the osmotic pressure differential, prior to completion of filtration, and there will be no osmotic pressure differential left to further draw in extracellular water to provide the desired increase in firmness.
It has been my observation that the addition of salt or other osmotically-active compounds in the concentrations prescribed by the Keustler et al patent causes not only water to be removed from the interior of the individual yeast cells, but also causes the release of soluble solids from the interior of the cells and other solids loosely held to the exterior of the cells. These substances tend to collect within the interstices of the filtering medium and interfere with filtering efficiency.
In one particular commercial yeast filtering operation employing rotary vacuum filters, the filtering medium is made up of a layer of starch particles deposited on a cloth or metal mesh on the surface of vacuum drum. The yeast is applied to the filtering medium on the surface of the drum, and the extracellular water is drawn through the filtering medium towards the interior of the drum to the extent possible due to the applied pressure differential. The resulting filter cake which comprises yeast having a solids content of 30% or greater, is normally removed from the drum by means of a cutoff knife. In normal commercial practice, the knife will be intermittently or continuously advanced towards the surface of the drum to remove the yeast cake and the very top layer of the starch-filtering medium. This top layer must be continuously removed in very fine cuts because it becomes plugged by the solid substances which are present in the liquid phase of the yeast suspension.
The amount of extracellular solids in a yeast suspension varies with many factors; however, one of the prime factors responsible for high levels is the addition of salt or other osmotically-active material to the cream yeast suspension. Thus, the achievement of the desired plasticity according to the Kuestler et al patent also releases solid substances which interfere with the efficiency of filtration. These interfering substances, when present in large quantities, tend to bind the filtering medium to a point at which the desired low moisture content for the filter cake cannot be obtained without the knife being advanced so rapidly into the starch coating that the amount of starch being cut off with the product is brought to an unacceptably high level. This requires more frequent replacement of the starch layer on the filter and also greatly reduces the feed rate of cream yeast to the filter and subsequent production of compressed yeast.
Accordingly, it would be desirable to have a process which retained the benefits of improved plasticity by treatment before filtration with an osmotically-active agent, while improving the rate of filtration and the life of the filtering medium. Moreover, it would be desirable to provide a process which improved the ultimate quality of the compressed yeast produced.